herbage as milk and meat means the slow depletion of the soil. In 

 addition to the nitrogen removed, with every 1,000 pounds of milk pro- 

 duced by cows on pasture, about 1.6 pounds of lime and 2 pounds of 

 phosphoric acid are carried away. With every ton of alfalfa hay 39 

 pounds of lime, 10.8 pounds of phosphoric acid, and 44.6 pounds of 

 potash are removed ; and the soil is depleted to that extent. Timothy 

 removes 5 pounds of lime, 3.6 pounds of phosphoric acid, and 13.4 

 pounds of potash in every ton of hay. 



This depletion of the soil of minerals simply cannot be avoided if 

 the crop is to be utilized. In the course of time these minerals will 

 have to be replaced. If they are not replaced, a gradual decline 

 in productivity may occur even without erosion. Fortunately, 

 replacement of minerals is not very expensive and pays well. 



Figure 1.— This crop of cotton has depleted the soil because it has permitted erosion. 



Organic matter is essential to soil, and nitrogen, an important food 

 element, is present in it. Soil-conserving and soil-building crops 

 must be considered chiefly in terms of the effect 

 Depletion of these crops have on the organic matter in soils. 

 Organic Mat- Where soil is covered with a sod the organic matter 

 ter is increased rather than diminished, but where a 



cultivated crop is grown, whether a legume or a 

 nonlegume, organic matter is consumed ; it is literally burned up. 



The destruction of organic matter is brought about by a process of 

 oxidation through the growth of micro-organisms. This process is 

 hastened bv the aeration of the soil that follows cultivation. The 

 more intensive the cultivation, therefore, especially in warm sections, 

 the greater the destruction of organic matter. Here soil erosion also 

 plays a great part. The organic matter is mainly in the surface soil. 

 It is this surface soil that is washed away, and the organic matter 

 goes with it. Erosion too is facilitated by the stirring of the soil. 

 Soil-depleting and soil-conserving crops should therefore be con- 

 sidered in relation to their effect on the organic matter as well as on 

 the minerals in the soil. Soil-building crops can effect a change onb" 

 in the amount of organic matter in the soil. 



It is certain that soil erosion plays a greater part than the mere 



